Study on the transports in transient flow over impulsively started circular cylinder using Lagrangian coherent structures

The material transport in transient flow over impulsively started circular cylinder is studied from Lagrangian point of view, in order to understand the nature of some complex transient phenomena. The transient flow is simulated numerically by CBS scheme with dual time stepping. Then, the transport processes during the development of separation bubbles and vortices are analyzed in terms of Lagrangian coherent structures (LCSs), in order to understand the transport and mixture in the transient phenomena and their contributions to the aerodynamic performance of circular cylinder. Results show that the transport processes in the transition flow are well described by the LCSs. In particular, at the beginning stage of formations of flow separation and vortex, the transports between main flow and separated flow are strong, and the aerodynamic performance of circular cylinder can be affected significantly. During the transition of flow states from symmetric flow to periodic vortex shedding, the dynamical behaviors of flow are different from that of periodic flow. Compared with Eulerian description, it can be seen that the Lagrangian description is much more available for the analysis of transient flow. More, the results are also instructive to the flow control from the viewpoint of nonlinear dynamics.